Presently, foams used in packaging, protective, and insulating materials are made from oil and natural gas derived thermoplastics such as alkenyl aromatic polymers (e.g., polystyrene) or polyolefins (e.g. polyethylene and polypropylene). Such polymers, designated herein “conventional polymers” or “conventional plastics,” do not undergo biodegradation and become a fixture of landfills and litter. On the other hand, biodegradable polymers are thermoplastics that are easily melt processed just like the conventional thermoplastics, but with the added attribute that they undergo hydrolytic decomposition or biodegradation in aerobic (such as composts) and anaerobic (such as landfills) environments where microbes break down the polymer to give primarily methane, carbon dioxide, organic residue called humus, and water. Biodegradable polymers can be made from petrochemical feedstock or, alternatively, from renewable biomass such as, for example, corn, sugarcane, wood, switchgrass or soybeans. Petrochemical based biodegradable polymers include, for example, various polyesters such as Biomax and Ecoflex. Biomass based polymers, also known as biopolymers, include, for example, polymers containing hydroxy acids such as polylactide or their esters such as polyhydroxyalkanoates. It should be noted, however, that not all biopolymers are biodegradable, and not all petroleum based polymers are non-biodegradable.
Recently, the advent of biomass derived polymers has shown that biodegradable thermoplastics having properties similar to those of conventional plastics can be prepared on a commercial scale. If means can be found to further improve the properties of biodegradable polymers (e.g. mechanical strength, elongational viscosity, stability over a wide temperature range, compatibility with conventional and other biodegradable polymers, etc.) then, by replacing conventional polymers with biodegradable polymers, the environmental aspects of solid plastic waste can be largely mitigated and, for certain applications, can be completely eliminated. The biodegradable foams can be used for various packaging applications where foams made from conventional polymers are currently employed.
Foams are commonly manufactured as expanded beads, extruded sheets, or extruded boards. The difference between the expanded and extruded foams is that the extruded foams, in the form of continuous sheets or boards, are made in a single-step process; whereas, expanded foams, in the form of discrete, small-size pieces, are made in a multi-step process. Thus, the dimensions of expanded foam are much smaller than those of extruded foam. Furthermore, the expanded foams do not necessarily have to be in the form of beads or peanuts, but can also be made from pellets, rods, platelets, thin sheet or film. For the sake of convenience, the term “bead” or “pellets” will be used throughout this application to imply other shapes in which small, discrete particles of the polymer resin can be used to make expanded foams.
Generally, foams in the form of beads or sheets having a thickness of less than about one-half inch can be used to make packaging materials such as containers (e.g. cups, bowls, clamshells, picnic chests) for hot or cold beverages or food whereby the beads are fused or the sheet is thermoformed in a mold to form the packaging material of a desired shape. Such foams are also used as protective and cushioning materials for transportation of delicate or shock sensitive articles whereby the foam beads can be used as loose fill dunnage material and thin sheets can be used to provide protective wrapping.
Packaging and insulation foam products with a thickness greater than about 0.5 inch are called planks or boards. Such foam boards are produced in the desired shape and size by direct extrusion and cutting if needed, or by fusing the expanded foam beads. The foam boards can be used for protective packaging by die-cutting the boards to various shapes, for insulation, for dissipating mechanical energy as in automotive parts, or for cushioning floats. It is desirable that the foams used in such diverse applications be dimensionally stable; this characteristic is even more desirable for planks or boards.
Polymer foams are commonly made using a continuous process where a blowing agent laden molten resin is extruded under pressure through an appropriate die into a lower pressure atmosphere. Alternatively, a batch or staged process can be used, where small polymer beads (also called particles or pellets) are impregnated with blowing agent and then expanded by heating rapidly to a temperature near or above the glass-transition or crystal-melt temperature of the polymer-blowing agent system, or subjected to an external compressive stress at a temperature up to the glass-transition or crystal-melt temperature of the polymer-blowing agent system. Presently, physical blowing agents more commonly used for making thermoplastic polymer foams are hydrocarbons, chlorinated hydrocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, or combinations thereof. Hydrocarbons with three or more carbon atoms are considered volatile organic compounds (VOCs) that can lead to formation of smog. Furthermore, some halogenated hydrocarbons are either VOCs or may have high ozone depletion potential (ODP) or global warming potential (GWP), or may be hazardous air pollutants (HAPs) and, at times, may fall into more than one of these categories. Therefore, the use of hydrocarbon and halogenated hydrocarbon blowing agents for preparing polymeric foams is not preferred environmentally and imposes many limitations on the manufacturing process, thus complicating and significantly increasing the cost of manufacturing. In efforts to make biodegradable polymer foams (beads or sheets), the conventionally used blowing agents, such as VOCs, have been the obvious choice, albeit such uses are associated with the same environmental concerns as noted above. It is therefore desirable to minimize or eliminate altogether the use of such compounds as blowing agents for preparing biodegradable polymer foams.
Methyl formate is classified as a non-VOC (Federal Register, Volume 69, Number 228, Nov. 29, 2004), is non-HAP, has zero ODP, and negligible GWP. U.S. Pat. No. 6,753,357 to Kalinowski et al., describes the use of methyl formate to produce stable, rigid isocyanate/polyol based polyurethane foams. It is noted, however, that such polyurethane foams are thermoset, so as to be made via a cross-linking and curing process. The dimensional stability or instability imparted to the final polyurethane foam product by the nature of the blowing agent therefore is quite different than in the case of thermoplastic polymer foams.
U.S. Pat. No. 3,358,060 to Ohsol, which is incorporated in its entirety herein by reference thereto, is directed to a process for forming foam bodies with a thickness of up to four inches, by premixing polymer pellets with a minor amount of absorbent, which has been charged with the desired amount of foaming agent, and then melt extruding the mixture in the conventional way. Ohsol describes that any conventional absorbent can be used to entrap or hold the volatile liquid (i.e., the foaming agent). Alternatively, to avoid use of the absorbent, Ohsol describes a method in which polymer beads impregnated with the volatile liquid are fed into the extruder to produce thick foam bodies. A number of volatile liquids, including methyl formate, are proposed as foaming agents. Ohsol discloses that suitable thermoplastic resins include cellulose ethers and esters, for example, ethyl cellulose and cellulose acetate. The extruded foam body from Ohsol's process tends to develop surface irregularities and corrugated surfaces, which require further processing. Accordingly, Ohsol discloses removing a portion of the surface of the extruded board, which contains the surface irregularities or corrugated surface, with a cutting member. While cutting the surface of the board may remove any surface irregularities, one drawback is that it also creates a large number of open cells on the surface of the board.
U.S. Pat. No. 3,085,073 to Lintner et al., which is incorporated in its entirety herein by reference thereto, discloses the production of a heat expandable thermoplastic resin in granular form via a diffusion/solvent washing technique using a blowing agent mixture. The blowing agent mixture comprises a solvent blowing agent, such as methyl formate, and a non-solvent blowing agent, such as pentane, with the requirement that the solvent and non-solvent components be miscible. The process requires the step of extracting the solvent blowing agent with a suitable liquid solvent such that the amount of solvent blowing agent is reduced to less than two weight percent, without affecting the amount of non-solvent blowing agent in the granules. The impregnation and extraction of blowing agents are carried out at room temperature or at a temperature below the glass transition temperature of the polymer.
U.S. Pat. No. 5,422,053 to Sterzel, which is incorporated in its entirety herein by reference thereto, is directed to a process for injection molding foamed parts, which include polylactide. The process includes melt extruding and then pelletizing a mixture of polylactide and 10 to 30 percent by weight of a solvent, such as methyl formate, followed by drying the pellets at room temperature and allowing the polylactide to crystallize. Alternatively, the crystalline polylactide pellets are obtained by mixing the pellets with the solvent at room temperature and allowing sufficient time for the solvent to diffuse into the pellets. The solvent-laden polylactide pellets are then fed into an injection-molding machine to make the foamed part. U.S. Pat. No. 5,348,983 to Sterzel, which is incorporated in its entirety herein by reference thereto, is directed to rigid polylactide moldings obtained by fusing foamed amorphous polylactide granules with finely divided, unfoamed, semi-crystalline polylactide particles. The unfoamed polylactide particles are devoid of any blowing agent.
Therefore, a need exists not only to make thermoplastic foams with minimum or no impact on air quality and which minimize the accumulation of solid waste on our planet, but also to produce these thermoplastic foams efficiently. Thus, environmentally benign polymers which undergo biodegradation and do not contribute to solid waste, and blowing agents employing methyl formate and environmentally friendly co-blowing agents, preferably non-VOC co-blowing agents, as components of the blowing agent blend, provide the necessary ingredients to produce stable biodegradable and low-emission polymeric foams.